Thick-film thermal print head and its manufacturing method

ABSTRACT

The thermal printhead ( 1 ) includes an insulating substrate ( 2 ), a heating resister ( 5 ) formed on the substrate ( 2 ), a first glass coat layer ( 7 ) formed on the substrate ( 2 ) for covering the heating resister ( 5 ), and a second glass coat layer ( 8 ) formed on the first glass coat layer ( 7 ). The heating resister ( 5 ) has a centerline average roughness not greater than 0.3 μm. The first glass coat layer ( 7 ) has a centerline average roughness not greater than 0.1 μm.

TECHNICAL FIELD

The present invention relates to a thick-film thermal printhead.Specifically, the present invention relates to a thick-film thermalprinthead including a very hard glass layer for protection of theheating resister. Further, the present invention also relates to amethod for manufacturing such a thick-film thermal printhead as theabove.

BACKGROUND ART

An example of a prior art thick-film thermal printhead is shown in FIG.5. The illustrated thermal printhead includes an insulating substrate51, a glaze layer 52 formed on the substrate 51 for heat reservation,and a conductor pattern 53 formed on the glaze layer 52. The Conductorpattern 53 includes a common electrode, individual electrodes and so on.The thermal printhead further includes a heating resister 54electrically connected with the conductor pattern 53, and a first glasscoat layer 55 for protection of the heating resister 54, the conductorpattern 53 and the glaze layer.

In addition to these constituent elements described above, the aboveprior art thermal printhead further includes a second glass coat layer56 formed on the first glass coat layer 55.

The second glass coat layer 56 is made of a highly strong glassmaterial. Such an arrangement as described above is adopted in order toprovide reliable protection to the heating resister 54 and others.

According to the prior art thermal printhead, the heating resister 54 isformed by first printing and then baking a predetermined resister pasteon the glaze layer 52. Specifically, the paste material is a mixture ofruthenium oxide, a glass frit and a solvent. The glass frit has anaverage grain size of about 5 μm.

The first glass coat layer 55 is formed for example of an amorphous leadglass containing about 26.5% resin material and about 73.5% glassmaterial. A glass paste for forming the glass layer 55 is a mixture of aglass frit and a solvent. The glass frit has a maximum grain size ofabout 10 μm.

The prior art thermal printhead is known to have a problem in thefollowing point. Specifically, as described as above, the average grainsize of the glass frit contained in the resister paste is about 5 μm.The heating resister 54 made from such a resister paste has a surfaceroughness expressed as centerline average roughness Ra of about 0.6 μm,which is a relatively large value. Next, the maximum grain size of theglass frit contained in the glass paste is about 10 μm as has beendescribed.

The glass coat layer 55 made from such a glass paste has a surfaceroughness expressed as the centerline average roughness Ra of about 0.2μm, which is a relatively large value.

As will be understood easily, if the centerline average roughness Ra onthe surface of the heating resister 54 has a large value, the centerlineaverage roughness Ra on the surface of the first glass layer 55 also hasa large value (i.e. the first glass coat layer 55 has a poor state ofsurface). Under such a circumstance, if the second glass coat layer 56is subjected to an impact force and so on, there is a possibility thatstress concentration occurs in a specific location of the second glasscoat layer 56. As a result, the second glass coat layer 56 may develop acrack for example, or the second glass coat layer 56 may flake off thefirst glass coat layer 55.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a thick-film thermalprinthead capable of eliminating or reducing the problem describedabove. In order to achieve the object, the present invention makes useof the following technical means.

A thermal printhead provided by a first aspect of the present inventioncomprises an insulating substrate, a heating resister formed on thesubstrate, a first glass coat layer covering the heating resister andformed on the substrate; and

a second glass coat layer formed on the first glass coat layer, whereinthe heating resister has a centerline average roughness not greater than0.3 μm.

According to a preferred embodiment of the present invention, the firstglass coat layer has the centerline average roughness not greater than0.1 μm.

Preferably, the heating resister is formed from a paste materialcontaining a glass frit having an average grain size not greater than 2μm.

Further, the first glass coat layer may be formed from a paste materialcontaining a glass frit having an average grain size not greater than1.5 μm.

Preferably, the glass frit has a maximum grain size not greater than 6μm.

According to a second aspect of the present invention, there is provideda method for making a thermal printhead including an insulatingsubstrate, a heating resister formed on the substrate, a first glasscoat layer covering the heating resister and formed on the substrate,and a second glass coat layer formed on the first glass coat layer. Themethod comprises the steps of forming the heating resister on thesubstrate, forming the first glass coat layer, covering the heatingresister, and on the substrate, and forming the second glass coat layeron the first glass coat layer, wherein the heating resister is formedfrom a paste material containing a glass frit having an average grainsize not greater than 2 μm.

According to a preferred embodiment of the present invention, the abovemethod further includes a step of printing and baking the pastematerial.

Preferably, the first glass coat layer is formed from a paste materialincluding a glass frit having an average size not greater than 1.5 μm.Preferably, the glass frit has a maximum grain size not greater than 6μm.

The second glass coat layer can be formed by spattering.

Other features and advantages of the present invention will becomeclearer from an embodiment to be described with reference to theattached drawings.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 is a plan view showing a principal portion of a thick-filmthermal printhead according to the present invention.

FIG. 2 is a sectional view taken in lines II—II in FIG. 1.

FIG. 3 is a graph showing a relationship between an average grain sizeof a glass frit contained in a resister paste and a centerline averageroughness Ra on a surface of a heating resister.

FIG. 4 is a graph showing a relationship between the centerline averageroughness Ra and a rate of flaking off failure occurred on a secondglass coat layer.

FIG. 5 is a sectional view showing a principal portion of a prior artthick-film thermal printhead.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to FIG. 1 - FIG. 4.

FIG. 1 and FIG. 2 show a principal portion of a thick-film thermalprinthead (indicated wholly by numeral code 1) according to a preferredembodiment of the present invention. The thick-film thermal printhead 1includes an insulating substrate 2 (FIG. 2) made of a ceramic. Thesubstrate 2 has an upper surface formed with a glaze layer 6 for heatreservation. The glaze layer has an upper surface formed with a wiringpattern including a common electrode 3 and a plurality of individualelectrodes 4.

As shown in FIG. 1, the common electrode 3 has a plurality of teeth-likeelectrode portion 3 a (hereinafter simply called the “teeth”). Theseteeth 3 a are disposed alternately with the individual electrodes 4,with each of the individual electrodes 4 partially sandwiched between apair of mutually adjacent teeth 3 a. Each of the individual electrodes 4has an end portion formed with a bonding pad 4 a. These bonding pads 4 aare electrically connected with a drive IC (not illustrated).

As shown in FIG. 1, the upper surface of the glaze layer 6 is formedwith a straight-line heating resister 5 electrically connecting theteeth and the individual electrodes 4. The heating resister 5 includes aplurality of regions H (only one is shown in FIG. 1) each defined by apair of mutually adjacent teeth 3 a. Each of the regions H serves as aheating dot.

As shown in FIG. 2, the upper surface of the glaze layer 6 is formedwith a first glass coat layer 7, covering the common electrode 3,individual electrodes 4 and the heating resister 5. The first glass coatlayer 7 has an upper surface formed with a second glass coat layer 8having a high hardness and covering the first glass coating layer 7.

Next, the description will cover a method for making a thick-filmthermal printhead 1 having the constitution as described above.

First, a glaze layer 6 is formed by applying and baking a glass materialon an upper surface of a substrate 2. Then, a common electrode 3 andindividual electrodes 4 are formed on the glaze layer 6. The formationof these electrodes are made by first printing a predetermined patternof resinated gold on the glaze layer 6, then baking the printed pattern,and then etching unnecessary portions off the baked pattern.

Thereafter, as shown in FIG. 1, a heating resister 5 is formed acrossthe common electrode 3 and the individual electrodes 4. The formation ofthe heating resister is made by printing and baking a pattern ofresister paste on the glaze layer 6.

The resister paste for the formation of the heating resister 5 is amixture of ruthenium oxide, a glass frit and a solvent. The glass frithas an average grain size not greater than 2 μm. By using a glass frithaving such a small average grain size as the above, a remarkably smoothsurface can be achieved in a finished heating resister 5. Specifically,the heating resister 5 has a surface centerline average roughness Ra notgreater than 0.3 μm. The heating resister 5 has a maximum thickness ofabout 9 μm.

After the formation of the heating resister 5, a first glass coat layer7 is formed, covering the common electrode 3, the individual electrodes4 and the heating resister 5. The formation of the first glass coatlayer is made by printing and baking a pattern of glass paste. The glasspaste is a mixture of a glass frit and a solvent. The glass frit has anaverage grain size not greater than 1.5 μm or has a maximum grain sizenot greater than 6 μm. Therefore, the finished glass coat layer 7 has aremarkably smooth surface. Specifically, the glass coat layer 7 has asurface roughness as expressed in the centerline average roughness Ranot greater than 0.1 μm. The glass coat layer 7 has a thickness of about6 μm.

After the formation of the first glass coat layer 7, a second glass coatlayer 8 having a high hardness and covering an upper surface of theglass coat layer 7 is formed by spattering. The second glass coat layer8 has a thickness of about 4 μm.

Generally, the second glass coat layer 8 obtained by spattering hasresidual stress. Under such a circumstance as this, if the surface ofthe first glass coat layer 7 is not sufficiently smooth (See FIG. 5),when the second glass coat layer 8 is subjected to an impact and so on,there is a possibility that stress concentration occurs in a specificlocation of the second glass coat layer 8. As a result, the second glasscoat layer 8 may develop a crack for example, or the second glass coatlayer 8 may flake off the first glass coat layer 7, resulting in afailure.

According to the thermal printhead 1 provided by the present invention,the surface of the first glass coat layer 7 is remarkably smooth. Thus,such problems as described above can be effectively prevented.

The inventors of the present invention conducted an experiment in orderto clarify relationship between an average grain size of the glass fritin the resister paste and the centerline average roughness Ra on asurface of the heating resister 5 formed from the resister paste. FIG. 3is a graph showing a result of the experiment. The graph shows that thecenterline average roughness Ra increases with increase in the averagegrain size of the glass frit.

According to the prior art thermal printhead, when the average grainsize of the glass frit is about 5 μm, the centerline average roughnessRa on the surface of the heating resister is about 0.6 μm. This statecorresponds to Point A in the graph. On the other hand, according to thepreferred embodiment of the present invention, the average grain size ofthe glass frit is not greater than 2 μm. As understood from the graph inFIG. 3, when the average grain size is 2 μm, the centerline averageroughness Ra is 0.2 μm (See Point B). Therefore, if the average grainsize is not greater than 2 μm, the centerline average roughness Ra canbe not greater than 0.2 μm.

Next, reference will be made to FIG. 4. A graph in FIG. 4 shows arelationship between the centerline average roughness Ra on the surfaceof the heating resister and the rate of flaking failure found in thesecond glass coat layer. (This graph is also based on the experimentconducted by the inventors.) As understood from the graph, the flakingrate increases when the centerline average roughness Ra increases. Inthe prior art, the centerline average roughness Ra is about 0.6 μm,resulting in about 10% flaking failure rate (See Point C). On thecontrary, when the centerline average roughness Ra is 0.2 μm, theflaking failure rate decreases to about 1% (See Point D). According tothe preferred embodiment of the present invention, since the centerlineaverage roughness Ra is 0.2 μm, the flaking failure rate can bedecreased to not greater than about 1%.

Thus far, a thick-film thermal printhead according to the preferredembodiment of the present invention and a method for making the samehave been described. The present invention however, is not limited bythe embodiments. For example, in the preferred embodiment, a glass frithaving a small average grain size is used in both of the resister pastefor forming the heating resister and the glass paste for forming thefirst glass coat layer. Alternatively, it is also possible to use theglass frit of a small average grain size only in one of the resisterpaste and the glass paste.

What is claimed is:
 1. A thermal printhead comprising: an insulatingsubstrate; a heating resister formed on the substrate; a first glasscoat layer formed on the substrate by printing and baking a glass pastefor covering the heating resister, the first glass coat layer beingformed from a paste material containing a glass frit having an averagegrain size not greater than 1.5 μm; and a second glass coat layer formedon the first glass coat layer by spattering; wherein the heatingresister has a centerline average roughness not greater than 0.3 μm. 2.The thermal printhead according to claim 1, wherein the first glass coatlayer has the centerline average roughness not greater than 0.1 μm. 3.The thermal printhead according to claim 1, wherein the heating resisteris formed from a paste material containing a glass frit having anaverage grain size not greater than 2 μm.
 4. The thermal printheadaccording to claim 1, wherein the glass frit contained in the pastematerial for the formation of the first glass coat layer has a maximumgrain size not greater than 6 μm.
 5. A method for making a thermalprinthead which comprises an insulating substrate, a heating resisterformed on the substrate, a first glass coat layer formed on thesubstrate for covering the heating resister, and a second glass coatlayer formed on the first glass coat layer, the method comprising stepsof: forming the heating resister on the substrate; forming the firstglass coat layer on the substrate by printing and baking a glass pastefor covering the heating resister, the glass paste including a glassfrit having an average grain not greater than 1.5 μm; and forming thesecond glass coat layer on the first glass coat layer by spattering;wherein the heating resister is formed from a paste material containinga glass frit having an average grain size not greater than 2 μm.
 6. Themethod according to claim 5, further comprising a step of printing andbaking the paste material for the formation of the heating resister. 7.The method according to claim 5, wherein the glass frit contained in theglass paste for the formation of the first glass coat layer has amaximum grain size not greater than 6 μm.